Microwave circuits are commonly used for many communications applications such as satellite communications. Microwave circuits are essentially wave guides that take the shape of a rectangular or cylindrical structure. Often, a plurality of waveguides are coupled together to form filter circuits. Communications satellites commonly use elliptic function filters.
To obtain sufficient performance, bridge couplings connecting non-sequential resonators are employed. The couplings for a bridge coupling often requires a capacitive (sign non-inverting) coupling rather than an inductive (sign inverting) coupling.
Capacitive bridge couplings are commonly formed from probes that are inserted into the coupling. The probes are typically supported with some form of low loss dielectric. Two problems are associated with probes. First, it is often difficult to fine tune the adjustment to remove the effects of variable tolerances. Second, for high frequencies above 20 GHz, such probes are difficult to implement.
Another type of capacitive coupling is an extremely narrow slit aperture. The width of the slits must be large enough so that the cut off frequency of the slit lies below the filter frequency. The iris resonance is the cut-off frequency of the slot. Because of this limitation, the slot height must remain extremely small. One problem with machining an extremely small slot is that the tolerance variation alone makes this type of slot uncontrollable and unpredictable in a manufacturing environment. With a thin slot, the fine tuning is difficult to implement.